1. Field of the Invention
The present invention relates generally to echo cancelers and more specifically to an acoustic echo canceler for loudspeaking telephones and the like for canceling acoustic echoes produced as a result of acoustic coupling between a loudspeaker and a microphone.
2. Description of the Related Art
Japanese Laid-Open Patent Specification 07-58673 discloses an acoustic echo canceler for loudspeaking telephones or teleconferencing systems. FIG. 1 illustrates a prior art digital acoustic echo canceler 102 incorporated in a loudspeaking telephone, which is connected via a hybrid circuit 101 to a telephone exchange line 100. Analog speech signal from the telephone line 100 is converted to a digital signal by an analog-to-digital converter 110 and applied to an adaptive filter 112 for producing a digital replica of an acoustic echo, or xe2x80x9cpseudo-echoxe2x80x9d. The digital speech signal is also applied to a digital-to-analog converter 111 for driving a loudspeaker 104 with an analog speech signal amplified by an amplifier 103. The original sound signal is reconstructed by the loudspeaker 104 and part of the sound energy is coupled to a microphone 105, amplified by amplifier 106 and converted to digital form by an analog-to-digital converter 113. This talker""s voice would be returned to the source as an echo. This echo signal is applied to a digital subtractor 114, where it is cancelled with the echo replica supplied from the adaptive filter 112.
Adaptive filter 112 includes an input buffer 120 which serves as a tapped delay line for the remote speech signal to produce a series of successively delayed digital tap signals. These tap signals are supplied to tap-gain multipliers 121 where they are weighted by respective tap-gain coefficients from tap-gain update circuits 123 and then summed together in an adder 122. Tapgain multipliers 121 and the adder 122 form a convolutional integrator. Each update circuit 123 includes a multiplier (correlator) 124, an adder 125 and a unit delay element 126. Multiplier 124 of each update circuit is used to detect the correlation between the corresponding tap signal and a residual echo which appears at the output of subtractor 114. Adder 125 provides summation of the correlation output of multiplier 124 with the output of the delay element 126 to produce a tap-gain coefficient.
Since the echo cancelling effect by the adaptive filter is not sufficient to completely remove the acoustic echo, the residual echo is detected by a level detector 115. A gain controller 116 is responsive to the detected level of the residual echo for controlling a gain adjustment circuit 117, which is interposed in the circuit coupling the output of subtractor 114 to the hybrid circuit 101 via a digital-to-analog converter 118. When the amplitude of the output of subtractor 114 is lower than some critical level, the level detector 115 directs the gain controller 116 to control the gain adjustment circuit 117 to reduce its gain, so that the acoustically coupled residual echo is prevented from returning to the talker.
However, one disadvantage of the prior art is that the amount of computations involved in the gain adjustment circuit 117 is substantial. Another disadvantage is that, since the gain adjustment is determined exclusively by the residual echo, the range of adjustment is still insufficient to cover large residual echoes.
It is therefore an object of the present invention to provide an acoustic echo canceler which does not require time-consuming, complex computations.
Another object of the present invention is to provide an acoustic echo canceler capable of canceling large residual echoes.
According to a first aspect, the present invention provides an acoustic echo canceler for a speech communication system including a loudspeaker and a microphone acoustically coupled with the louspeaker through a feedback path. The acoustic echo canceler comprises an adaptive filter including a tapped delay line for receiving a remote speech signal to produce a sequence of successively delayed tap signals and a convolutional integrator for respectively weighting the tap signals with filter coefficients and summing the weighted tap signals to produce an acoustic echo replica of the remote speech signal, the adaptive filter updating the filter coefficients with a residual echo and restricting the filter coefficients to those which correspond to an echo component of the remote speech signal and restricting the tap signals to those which correspond to the restricted filter coefficients. The output signal of the microphone is combined in a subtractor with the acoustic echo replica, producing the residual echo. An attenuator is provided for receiving an output signal of the subtractor. Comparator circuitry compares the output signal of the subtractor with the restricted tap signals of the adaptive filter for increasing the attenuation of the attenuator when the restricted tap signals are greater in magnitude than the output signal of the subtractor and decreasing the attenuation when the restricted tap signals are smaller in magnitude than the output signal of the subtractor.
According to a second aspect, the present invention provides an acoustic echo canceler for a speech communication system including a loudspeaker and a microphone acoustically coupled with the louspeaker through a feedback path. The acoustic echo canceler comprises an adaptive filter including a tapped delay line for receiving a remote speech signal to produce a sequence of successively delayed tap signals and a convolutional integrator for respectively weighting the tap signals with filter coefficients and summing the weighted tap signals to produce an acoustic echo replica of the remote speech signal, the adaptive filter updating the filter coefficients with a residual echo and restricting the filter coefficients to those which correspond to an echo component of the remote speech signal and restricting the tap signals to those which correspond to the restricted filter coefficients. The output signal of the microphone is combined in a subtractor with the acoustic echo replica, producing the residual echo. A first attenuator is provided for suppressing small amplitude components of the residual echo and a second attenuator receives the output signal of the first attenuator. Comparator circuitry compares the output signal of the first attenuator with the restricted tap signals of the adaptive filter for increasing the attenuation of the second attenuator when the restricted tap signals are greater in magnitude than the output signal of the first attenuator and decreasing the attenuation of the second attenuator when the restricted tap signals are smaller in magnitude than the output signal of the first attenuator